There has been a continuing interest in improving the electrode active material quality in lead-acid storage batteries to increase energy efficiency and operational life. Various additives, commonly referred to as expanders, have been used particularly in the paste for making negative battery plates. Expanders improve the efficiency of utilization of the active materials, particularly, the high rate, low temperature discharge capacity and the operational life of the battery.
In a typical SLI battery (automotive starting, lighting and ignition), efficiency of utilization of the negative electrode active material is generally substantially less than the positive. For example, in 20-hour rate discharge of 50 A-hour SLI batteries, the utilization efficiency of negative active material is about 53% to 59% of the positive active material. This low efficiency of negative electrodes is due to various factors such as the design of the battery, that is, the number of negative plates in a cell generally exceeds the number of positives by one, the need to maintain some minimum grid thickness for handling, and difficulties in preparing and controlling the active material morphology. While the electrical conducitivity of negative active material is higher than that of positive active material, the expected higher utilization efficiency of negative material has been to-date offset by an active material morphology in which the surface area of the negative active material is an order of magnitude less than for the positive. The effective surface area of the active material is known to control the cold performance (low temperature high rate discharge capacity) and for this reason an excess amount of negative active material is provided. This condition, causes the cell to be limited by the positive electrodes under normal operation. Under conditions such as very low temperature and higher discharge rates, however, the cell becomes negative-limited. Examination of failed batteries usually reveals negative plates to be in exceptionally good condition at the time of failure, the end of battery life having been caused by failure of the positive plates.
Efforts have been made to improve the performance of negative electrodes by incorporating the additives as suggested in U.S. Pat. Nos. 3,523,041, 3,446,670, 3,481,785 and 3,480,478, and British Pat. No. 1,307,221. Others have suggested preparing a low density paste, as for example, U.S. Pat. Nos. 3,496,030, 3,702,265 and 3,765,943. While some of the above patents disclose improved cold performance of the negative active material, none has obtained improved material utilization efficiency over a wide range of temperatures and discharge currents or an effective means of reducing metallic lead content in sponge lead electrode.
More recently, published West German patent application No. 2,724,839 has disclosed the addition of starch-coated glass fibers in an amount ranging from 0.25 to 0.5% of the active mass. The fiber lengths disclosed therein range from 8 to 15 mm and the diameters from 8 to 15 microns.